A clamping device of this type, essentially consist of a head piece with a drive element arranged thereon. The linearly movable actuator and the tensioning arm can be actuated from the drive. The linear adjustment is converted into a pivoting motion of the tensioning arm of the device by means of an adjusting mechanism arranged in the head piece. The linearly movable actuator and tensioning arm usually can be adjusted into an over-center position in order to also preserve their clamped position in case the operating medium for the drive fails for whatever reason. However, if the operating medium fails, it is also not readily possible to loosen the tensioned clamping device from its over-center position. This is the reason why clamping devices of this type are usually equipped with elements that make it possible to reset or unlock the clamping devices from their over-center positions. In this respect, we refer to the following publications: EP 1 060 839 A2, U.S. Pat. No. 6,557,841 B2, DE 196 16 441 C1 and DE 20 2007 010 690 U1. The unlocking elements of these clamping devices are all arranged in the upper region of the head piece in the pivoting range of the tensioning arm. Namely, they are in alignment with actuators of the adjusting mechanism that extends underneath the locking elements in the head piece.
DE 196 45 778 C1 illustrates a clamping device of the initially cited type. This clamping device consists of a toggle lever mechanism that is provided with a linearly movable actuator and with a pivot shaft for a tensioning arm. A means to unlock an over-center position is assigned to the toggle lever mechanism. The means consist of a control element that is arranged on the actuator (referred to as a stopping face in the publication) and a rocker that can be brought into adjusting contact with the control element (referred to as a control pin in the publication).